Analysis and Design of Zero-Voltage-Switching Current-Fed Isolated Full-Bridge Dc/Dc Converter

Transcription

1 EEE PEDS 011, Sigapore, 5 8 December 011 alysis ad Desig of ZeroltageSwitchig CurretFed solated FullBridge Dc/Dc verter Prasaa U R, Member, EEE, ad kshay K Rathore, Member, EEE Electrical ad uter Egieerig Natioal Uiversity of Sigapore , Sigapore elepur@us.edu.sg, eleakr@us.edu.sg bstractthis paper presets steadystate aalysis ad desig of curretfed fullbridge dc/dc coverter with activecla. The coverter utilizes the eergy stored i the trasformer leakage, aided by its magetizig iductace to maitai zerovoltageswitchig of Hbridge switches over wide rage of iput voltage ad load variatio. ctivecla clas the voltage across the switches at turoff, allowig the selectio ad use of low voltage devices with low ostate resistace. additio, this small ad low ratig activecla circuit results i zerovoltage trasitio circuit aidig i achievig zero voltage switchig of the devices. ll switches udergo soft turo ad switchig losses of the coverter are reduced. t iroves the coverter efficiecy ad allows high switchig frequecy operatio, which results i coact, light weight, ad low cost system.. NTRODUCTON Curretfed coverters are pickigup attetio i applicatios requirig higher voltage coversio ratio ad which are sesitive to iput curret ripple. lso, it has bee observed that for highfrequecy (HF switchig operatio, curretfed topologies ca provide wide rage softswitchig while maitaiig higher efficiecy coared to voltagefed PWM ad resoat coverters for iput voltage variatio of 1: coditioig power from full load dow to light load [1]. Curretfed topologies have bee justified for fuel cells applicatios [3]. ow ripple curret magitude results operatig poit stability as well as fuel savig to ehace the overall system efficiecy ad to reduce the eergy cost. case of PV, the same results i better PV power utilizatio, stable MPPT, ad possibly low payback period. Curretfed full bridge coverter has bee aalyzed i [38]. Curretly, curretfed coverter topologies are cited i literature rapidly with the global boom i reewable eergy market with cocept of clea ad gree eergy icludig trasportatio to save the eviromet, specially fuel cells applicatios, i.e., fuel cell iverter, electrolyser, fuel cell vehicles (FCV etc. FCVs are o road i coutries US, ada ad Germay, ot oly cars but local trasportatio system as they have evirometal iact due to zero emissio. Similarly for PV microiverters, curretfed is a potetial cadidate amog choices to utilize PV power efficietly over wide badwidth of voltage ad curret with varyig solar isolatio ad teerature. Mostly, the literature o curretfed fullbridge topology is o hardswitchig. iterature o activecla based [45, 9 13] ZVS curretfed fullbridge topology is limited. this paper, authors do ot claim o the topology. The topology has bee discussed i [45]. However, the aalysis ad desig preseted are ew ad ever reported i literature. The objectives of this paper are to preset the aalysis ad desig of the highfrequecy isolated activeclaed curretfed fullbridge dc/dc coverter. The paper is orgaized as follow: Steadystate operatio ad aalysis of the coverter are described i Sectio. verter desig procedure with a desig exale is illustrated i Sectio. Simulatio results to verify the aalysis ad desig are preseted i Sectio V. Experimetal results are preseted i Sectio V.. OPERTON ND NYSS OF THE CONVERTER The followig assutios are made for the operatio ad aalysis of the coverter: 1 put iductor is large so that the curret through it is cosidered costat. Cla capacitor C a is large to maitai costat voltage across it. 3 ll cooets icludig switches ad diodes are ideal. Steadystate operatig waveforms are show i Fig.. Switches S 1 ad S 4 are operated by idetical gatig sigals, ad S ad S 3 are operated by same gatig sigals. Switchig frequecy of auxiliary switch S ax is double of that of mai switches. t is cotrolled by gatig sigal colemetary to the mai switches gatig sigals. Gatig sigals of switch pair S, S 3 are shifted i phase by 180 with gatig sigals of switch pair S 1, S 4 with a overlap. The overlap varies with duty cycle. Fixed frequecy duty cycle modulatio is used for cotrol. The operatio of the coverter durig differet itervals i a HF half cycle is explaied usig the equivalet circuits show i Fig. 3. Fig. 1 ctiveclaed ZVS curretfed fullbridge dc/dc coverter /11/$ EEE 39

2 terval 1 (Fig. 3a; t o < t < t 1 : this iterval, all four mai switches S 1 ~ S 4 are ON. uxiliary switch S ax is off. put iductor is storig eergy. Power is trasferred to the load by the output filter capacitor C o. Trasformer magetizig curret circulates through its leakage iductace, give by i i m ' (1 where i is trasformer iput or leakage iductace curret, i p ' is magetizig curret reflected to primary side ad is the peak value of magetizig curret, give by ( f s ( m ' ltage across the auxiliary capacitor C a is V V i (3 (1 D ltage across the auxiliary switch is V V (4 (1 D Duty ratio of mai switches D T o /T s ; T o mai switch coductio time ad T s switchig period. im. (tt (6 m is the peak curret through magetizig iductace (o secodary side. Curret through the switch S is give by i vs ( /. ( t t (7 The auxiliary cla capacitor curret i decreases liearly. terval (Fig. 3b; t 1 < t < t : t t t 1, mai switches S ad S 3 are tured off. put boost iductor 1 curret ( i diverts to the auxiliary circuit path causig zero curret through all mai switches. The magetizig curret flows through leakage iductace, atiparallel diodes D 1, D 4 of mai switches S 1 ad S 4. Therefore, switch currets through S 1 ad S 4 quickly dips to egative, which is equivalet to peak value of the reflected magetizig curret. Device capacitaces C ad C 3 of mai switches S ad S 3 start chargig ad auxiliary switch subber capacitor C ax starts dischargig liearly. Rectifier diodes are reverse biased ad power is still trasferred to the load by filter capacitor. The same costat curret ( flows through magetizig iductace. t the ed of this iterval, voltages across the mai switch S ad auxiliary switch S ax reach V (t V S3 (t V o / ad V (t V V o /, respectively. terval 3 (Fig. 3c; t < t < t 3 : This iterval is very small. Subber capacitors, partially charged i iterval, are still goig through chargig ad dischargig. The mai switch voltages v ad v S3 icreases from V o / to V. positive voltage equal to (V V o / appears across the trasformer leakage iductace ad curret through it, i rises liearly. Output voltage V o appears across the magetizig iductace m ad curret through it starts icreasig liearly. Rectifier diodes DR 1 ad DR 4 are forward biased ad start coductig whe the leakage iductace curret i rises above i ' ad power is trasferred to the load. The leakage iductace curret i is give by vs ( V / o i. ( t t (5 The magetizig iductace curret i m is give by V i (1 D (1 D (1 D V o V i (1 D V o Fig.. Steadystate operatig waveforms of curretfed fullbridge dc/dc coverter with activecla. 40

3 t the ed of this iterval, the auxiliary switch subber capacitor C ax is discharged coletely to zero ad C ad C 3 are charged to its full voltage, equal to V. Fial values are: v x (t 3 v (t 3 0; vs ( t3 vc ( t3 V (1 D terval 4 (Fig. 3d; t 3 < t < t 4 : this iterval, the atiparallel body diode D ax of the auxiliary switch S ax starts coductig ad S ax ca be gated for ZVS tur o. eakage iductace curret i is icreasig with the slope of [(V V o // ]. this iterval, switch curret (through S 1 ad S 4 chages directio to positive magitude. Curret through the magetizig iductace is icreasig with the same slope. Trasformer leakage iductace curret i is give by V ( / i i ( t3. ( t t3 (8 Curret through the switch S 1 is give by V ( / ( t3. ( t t3 (9 Magetizig iductace curret is give by V i i ( t o m m 3. (tt3 (10 m uxiliary capacitor curret durig this iterval is decreasig ad is give by V i, peak. (tt3 (11 t the ed of this iterval, i.e., t t 4, i reaches zero, i reaches i ad also switch curret reaches i reaches i. Fial values are: i (t 4 i ; i (t 4 0; i (t 4 i. terval 5 (Fig. 3e; t 4 < t < t 5 : this iterval, the auxiliary switch S ax is tured o with ZVS. Curret i icreases above i with the same slope as iterval 4 ad curret i decreases liearly (egative directio. Curret i m is icreasig with the same slope as iterval 4. The equatios for this iterval are V i i. (tt 4 (1 S i i i 1 (13 V i i i. (tt4 (14 Peak value of the switch curret is give by:, peak i '. ( 1 D fs m (15 t the ed of this iterval, curret i rises to egative peak i ad therefore the currets i ad i reaches their peak value. Fial values: i (t 5,,peak ; i (t 5,peak ; i (t 5,peak. terval 6 (Fig. 3f; t 5 < t < t 6 : The auxiliary switch S ax is tured off at t t 5. Curret i charges C ax ad discharges C ad C 3. The leakage iductace resoates with subber capacitors C a1 ad C C 3. This period is very short. ad the series iductor curret icreases a very little i this iterval. The resoat frequecy is give by 1 (16 ( C 3 1 ltage across the capacitor C or switch S is give by v S V v (17 where the voltage across the switch S ax (or capacitor C ax is give by v, peak si( ( t t5 (18 ( C3 1 i, peak cos( ( t t5 (19 Mai switch curret is give by, peak cos( ( t t5 (0 t the ed of this iterval, C ad C 3 discharge to V o / ad C ax charges to (V V o /. Fial values are (eglectig small icrease i curret i this short iterval: v (t 6 V V o /; v (t 6 V o /; terval 7 (Fig. 3g t 6 < t < t 7 : Curret i is still chargig C ax ad dischargig C ad C 3 i a resoat fashio. t is short time iterval ad the curret i decreases a very little i this iterval. t the ed of this iterval, the capacitors C ad C 3 discharges coletely to zero ad capacitor C ax charges to its iitial value. Fial values are: v (t 7 0; v (t 7 V. terval 8 (Fig. 3h; t 7 < t < t 8 : this iterval, atiparallel body diode D of mai switch S ad D 3 of mai switch S 3 start coductig ad ow S ad S 3 ca be gated for ZVS tur o. Curret i decreases with a egative slope of [V o /( ]. i ( t7. ( t t7 (1 i i i ( This iterval eds whe curret i i. Fial values are: i (t 6 0; i (t 6 i. terval 9 (Fig. 3i; t 8 < t < t 9 : this iterval, switches S ad S 3 are tured o with ZVS. Currets i ad i S3 start icreasig ad the curret i is decreasig with the same slope. Curret i is trasferred to the switches S ad S 3. The iterval eds whe curret i equals to the curret i m. Switch S ad S 3 curret reaches to i ad S 1 ad S 4 curret reaches to i. i i. (tt (3 8 is. ( t t8 (4 i. (tt8 (5 Fial values: i (t 9 i ; i (t 9 i ; i (t 9 i p (t 9. 41

4 Ci ii x i i v B B m im 1: R Ci i ii x C1 S3 i vb B S4 C3 C4 m im 1: R (a (i Fig. 3. Equivalet circuits durig differet itervals of operatio of the proposed coverter for the waveforms show i Fig.. (b For the ext half cycle, the itervals are repeated i the same sequece with other symmetrical devices coductig to colete the full HF cycle. The aalysis is doe to obtai the desig equatios to desig ad select the cooets as well as to evaluate the coverter s performace theoretically. Based o the above aalysis, the desig equatios for the coverter were derived ad preseted i the ext sectio.. DESGN OF THE CONVERTER Ci i ii x (c i vb B m im 1: R this sectio, desig procedure is illustrated by a desig exale with a coverter of followig specificatios: put voltage V i to 41 V, output voltage V o 350 V, output power P o 1 kw, switchig frequecy f s 100 khz. (1 verage iput curret is i P o /(ηv i. ssumig a efficiecy η of early 90%, i 50. Ci Ci i i ii x ii x (d i vb B (e i vb B m im 1: m im 1: R R ( D max is selected at miimum iput voltage V i V ad full load based o switch voltage ratig V SW,max usig Dmax 1 (6 Vsw, max For V SW(max 55 V, D max 0.8. (3 Values of boost iductor are give by (V i (D0.5/[(Δ i (f s ] (7 where Δ i is the boost iductor ripple curret. For Δ i 1, 66 μh. Maximum voltage across the iductors V V i 33 V. (4 Switch curret ratigs: pproximate value of rms curret through the mai switches,rms ca be give by Ci i ii x (f i vb B (g (h m im 1: R 1/ i, rms D (8 4 RMS curret through the auxiliary switches is give by ( [( ] 1/, rms i 1 D / 1 (9 The values of,rms ad,rms are calculated to be 9. ad 7.1 respectively. Peak currets through mai switches S,peak 103 ad auxiliary switches,peak i,peak 55. verage curret through auxiliary switches as well as atiparallel diodes is give by (1 D ( (30, av i 4 Here,,av.75. verage curret through the mai switches,av i / 5. 4

5 (5 uxiliary capacitor: Substitutig i (3, V i V ad D 0.8, V 55 V. The value of auxiliary capacitor C a is C a,peak s ( D 1 4 π f ΔV / 3 (31 Peak curret through C a is,peak i 55. For a ripple voltage of ΔV V, C a 8 μf. RMS curret through auxiliary capacitor is, rms, peak ( 1 D 3 (3 Here,,rms uxiliary capacitor carries a curret of 00 khz (twice the switchig frequecy. (6 Output rectifier diodes: verage rectifier diode curret is give by DR,avg P o /(V o (33 Here, DR,avg ltage ratig of rectifier diodes, V DR V o 350 V. (7 Output capacitor: Value of output filter capacitor C o is Ts ( 0 TDR C o (34 Δ ΔV o llowable ripple i output voltage. C o 10 μf for ΔV o 0.7 V. ts voltage ratig is V o 350 V. (8 Subber desig: The equatio for the calculatio of subber capacitors is give by t f ( i (1 D ( C1 C4 C (35 Here, t f fall time of the switches durig turoff. C 1 C 4 C oss, ; C a1 (C 1 C 4 1 C oss,. Here, C oss, is the device capacitace of the selected mai switches, which ca be checked from their datasheet, the usig (35, subber capacitor C ax ca be calculated. egative peak of the magetizig curret. verage leakage iductace curret is a measure of power trasferred to the output. t is clear that its value is higher tha. i as derived i the aalysis Sectio of the coverter. Fig. 6 shows the waveforms of currets through auxiliary switch S ax ad cla capacitor C a. Their frequecy of operatio is 00 khz, i.e. twice of the mai switches. Similar to mai switches, the atiparallel diode of the auxiliary switch coducts prior to coductio of the switch favorig its ZVS. Therefore, all the switches, mai ad auxiliary, are udergoig ZVS softswitchig. Their peak values are the sum of iput curret ad reflected peak magetizig curret. Fig. 7 shows the voltage waveforms across the leakage ad magetizig iductace of the highfrequecy trasformer. t is clear that the voltage across the magetizig iductace is the output voltage ad appears wheever the rectifier diodes are coductig. Due to capacitive output filter, it is free from duty cycle loss, voltage rigig ad secodary subber requiremets. ltage across the leakage iductace is very low. ow voltage results i low V ratig of the trasformer. V. SMUTON RESUTS The desiged coverter has bee simulated usig software package PSM 9.0 Simulatio results are illustrated i Figs. 48. Figs. 48 coicide well with the theoretically predicted waveforms. t verifies the steadystate aalysis of the coverter, preseted i Sectio. Fig. 4 shows the switch curret waveforms through S 1 ad S. The curret through S 3 is idetical to curret through S ad curret through S 4 is similar to curret through S 1. t is clear from Fig. 4 that the atiparallel body diode of the switch coducts before the starts coductig curret through it. t results i ZVS o of the switches. t verifies the desig of the coverter explaied i Sectio. Fig. 5 shows curret waveforms through leakage ad magetizig iductaces of the highfrequecy trasformer. Magetizig iductace curret follows the trapezoidal profile ad leakage iductace curret follows triagular profile. Their currets are idetical reflected curret wheever rectifier diodes are off, which is positive ad Fig. 4. Switch curret waveforms of curretfed fullbridge dc/dc coverter with activecla. Fig. 5. Trasformer leakage ad magetizig iductace curret waveforms of curretfed fullbridge dc/dc coverter with activecla. 43

6 Fig. 8 illustrates the voltage across the mai ad auxiliary switches. The voltage across them is claed at low voltage. The problem of high voltage stress, i.e., large turoff voltage spike is elimiated. ow voltage switches ca be used, which have low ostate resistace. Therefore, low coductio losses ad high efficiecy are expected. Fig. 6. uxiliary switch S ax ad cla capacitor C a curret waveforms of curretfed fullbridge dc/dc coverter with activecla. V. EXPERMENT RESUTS laboratory prototype of the curretfed isolated full bridge dc/dc coverter rated at 500 W as show i Fig. 9 was built i ad tested to demostrate the experimetal results to verify the aalysis. The coverter has bee tested for iput voltage V i V, output voltage V o 350 V, output power P o 500 W, switchig frequecy f s 100 khz. Trasformer secodary to primary turs ratio 8, ad leakage ad magetizig iductaces are 0.5 µh (referred to primary ad m 3 mh (referred to secodary respectively. Experimetal results are illustrated i Figs Figs. 101 clearly cofirm ZVS of mai ad auxiliary switches. voltage waveforms show i Fig. 10, gatig sigals (v gs are applied to mai switches after voltage across them (v ds reaches zero resultig i zero voltage turo. The same is true for auxiliary switch S ax as show i Fig. 11. The ZVS of mai ad auxiliary switches is also cofirmed by Fig. 1 sice atiparallel diode (egative is coductig before the switch starts coductig (positive. Curret through the trasformer leakage iductace i is show i Fig. 13. Fig. 7. ltage waveforms across the trasformer leakage ad magetizig iductaces of curretfed fullbridge dc/dc coverter with activecla. Fig. 9. Experimetal laboratory prototype of 500 W curretfed fullbridge isolated dc/dc coverter. Fig. 8. ltage waveforms across the mai ad auxiliary switches of curretfed fullbridge dc/dc coverter with activecla. Fig. 10. Experimetal voltage waveforms: gatetosource V gs ad draitosource V ds waveforms across the mai switch showig ZVS. 44

7 Fig. 11. Experimetal voltage waveforms: gatetosource V gs, ad draitosource V ds, waveforms across the auxiliary cla switch showig ZVS. Steadystate aalysis ad desig of curretfed fullbridge dc/dc coverter with activecla have bee preseted i this paper. Simulatio results usig PSM 9.0 are preseted to verify the aalysis ad desig. lete modebymode aalysis ad waveforms have bee reported, which has ever bee illustrated i literature. Experimetal results o a low power lab prototype have bee demostrated to show softswitchig operatio ad low voltage stress across the devices. The duty cycle of the mai switches is always kept greater tha 50% due to icreased circulatig curret through the auxiliary devices. t results i uwated coductio losses causig lower coverter efficiecy, particularly at partial load as the auxiliary circuit loss becomes coetitive to the losses i mai switches. REFERENCES Fig. 1. Experimetal switch curret waveforms: i is curret through mai switch ad i is curret through auxiliary switch S ax. Fig. 13. Experimetal curret waveform through trasformer leakage. The curret waveforms through the switches (Hbridge ad auxiliary cla ad the trasformer curret ca be superiosed o the theoretically predicted steady state waveforms ad the same obtaied by simulatio. t verifies the proposed aalysis ad desig. V. SUMMRY ND CONCUSON Curretfed topologies are suitable for low voltage higher curret applicatios. However, these topologies suffer from higher voltage stress across the power semicoductor devices. ctivecla helps cla the voltage across the devices ad low voltage devices ca be selected. t results i low coductio losses ad higher efficiecy. additio, activecla provides a path to circulate the eergy stored i trasformer leakage iductace. The recirculatio of eergy discharges the subber capacitors across the switches before they are gated for tuig o. t results i ZVS o of switches. Softswitchig permits high switchig frequecy operatio resultig i coact, low cost ad light weight system. [1]. K. Rathore,. K. S. Bhat, ad R. Orugati, alysis, desig, ad experimetal results of wide rage ZVS activeclaed type curretfed DC/DC coverter for fuel cells to utility iterface, EEE Trasactios o dustrial Electroics, vol. 59, 01, pp []. K. Rathore,. K. S. Bhat, ad R. Orugati, coariso of softswitched DCDC coverters for fuel cell to utility iterface applicatio, EEJ Trasactios o dustry pplicatios, vol. 18, No. 4, 008, pp [3] X. Kog, ad. M. Khambadkoe, alysis ad ilemetatio of a high efficiecy, iterleaved curretfed full bridge coverter for fuel cell system, EEE Trasactios o Power Electroics, vol., 007, pp [4] V. Yakushev, V. Meleshim, ad S. Fraidli, Full bridge isolated curret fed coverter with activecla, Proc. EEE pplied Power Electroics ferece ad Expositio, 1999, pp [5] E. S. Park, S. J. Choi, J. M. ee, ad B. H. Cho, softswitchig activecla scheme for isolated fullbridge boost coverter, Proc. EEE pplied Power Electroics ferece ad Expositio, 004, pp [6] W. Sog, ad B. ehma, Curretfed dualbridge DC DC coverter, EEE Trasactios o Power Electroics, vol., 007, pp [7] M. Mohr ad F. W. Fuchs, Claig for curretfed de/dc coverters with recovery of claig eergy i fuel cell iverter systems, Proc. Europea ferece o Power Electroics ad pplicatios (EPE, 007 pp.110. [8]. verberg, K. R. Meyer, ad. Mertes, Curretfed fullbridge coverter for fuel cell systems, Proc. EEE Power Electroics Specialists ferece, 008, pp [9] JT. Kim, BK ee, TW. ee, SJ. Jag, SS. Kim, ad CY. Wo, active claig curretfed halfbridge coverter for fuelcell geeratio systems, Proc. EEE Power Electroics Specialists ferece, 004, pp [10] S. Ha, H. Yoo, G. Moo, M. You, Y. Kim, ad K. ee, ew active claig zerovoltage switchig PWM curretfed half bridge coverter, EEE Trasactios o Power Electroics, vol. 0, 006, pp [11] S. J. Jag, C. Y. Wo, B. K. ee ad J. Hur, Fuel cell geeratio system with a ew active claig curretfed halfbridge coverter, EEE Trasactios o Eergy versio, vol., 007, pp [1] JM. Kwo, ad BH. Kwo, High stepup activecla coverter with iputcurret doubler ad outputvoltage doubler for fuel cell power systems, EEE Trasactios o Power Electroics, vol. 4, Ja. 009, pp [13]. K. Rathore,. K. S. Bhat, ad R. Orugati, alysis ad desig of activeclaed ZVS curretfed DCDC coverter for fuel cells to utility iterface applicatio, Proc. EEE teratioal ferece o dustrial ad formatio Systems, 007, pp